As the density of semiconductor devices increases and the size of circuit elements becomes smaller, the resistance capacitance (RC) delay time increasingly dominates the circuit performance. To reduce the RC delay, there is a desire to switch from conventional dielectrics to low-k dielectrics. These materials are particularly useful as intermetal dielectrics, IMDs, and as interlayer dielectrics, ILDs. However, low-k materials present problems during processing, especially during the processing of the conductive material used to make interconnects.
The conductive material is typically patterned and etched using high-energy plasma etch processes. The low-k materials are susceptible to damage from a plasma etch because they are softer, less chemically stable or more porous, or any combination of these factors. The plasma damage can manifest itself in higher leakage currents, lower breakdown voltages, and changes in the dielectric constant associated with the low-k dielectric material.
One example of low-k dielectric materials are the porous dielectrics. Porous dielectrics may be formed by incorporating a pore generating material (a porogen) into a low-k dielectric matrix. One example of a low-k material is a carbon-doped oxide or organosilicate glass (OSG). Examples of commercially available porous dielectrics include Dow Chemical's SILK product and JSR Corporation's JSR 5109. The dielectric constant of the porous material is a combination of the dielectric constant of air and the dielectric constant of the low-k matrix material. Silica based xerogels and aerogels, for example, incorporate a large amount of air in pores or voids, thereby achieving dielectric constants less than 1.95 with pores as smallas 5-10 nm.
Porous dielectrics are susceptible to damage from plasma etching and ashing processes used in deice fabrication. When there is an open pore in the dielectric, processing fluids in lap and polish and in thin film metallization can enter surface pores, thereby causing corrosion, mechanical damage, or an increase in the dielectric constant. Pore damage may also cause a surface that is preferably hydrophobic to become hydrophilic. A hydrophilic surface tends to absorb moisture, which further increases the dielectric constant.
Given the properties of porous dielectrics, conventional wet cleaning methods are particularly problematic. As cleaning fluids enter the pores, this causes higher leakage currents, lower breakdown voltages, and changes in the dielectric constant associated with the low-k dielectric material. In view of these and other problems, there is a need for improved low-k dielectric manufacturing methods.